An in situ cylindrical internal reflection infrared technique (CIR-FTIR) was used in order to understand the behavior of subcritical and supercritical hydrocarbons processing over a commercial Y-type zeolite (P-Octacat) at 475 degrees C and pressures up to 41.6 bar. The changes in the concentrations of the active catalytic sites were monitored in situ by CIR-FTIR during the catalytic cracking of n-heptane at subcritical and supercritical conditions. High-quality in situ spectra were acquired for the first time at such high temperatures and pressures. The results showed that the catalyst maintained higher activity when catalytic cracking was performed under the supercritical conditions of the hydrocarbon, whereas the subcritical conditions led to a rapid deactivation of the catalyst. The subcritical LR spectra showed dramatic reductions in the concentration of the acid sites and in the interactions of the acid sites with the olefinic and paraffinic products of the reaction. Supercritical fuel showed an extraordinary dense behavior within the pores of the zeolite, which was significantly higher than the values estimated by equations of state under the same conditions. Higher amounts of carbonaceous deposits under subcritical conditions were also observed. Finally, reactivation studies on a deactivated catalyst resulted in a partial regeneration of the catalyst under supercritical conditions, provided with a complete experimental proof by CIR-FTIR.